a) Field of the Invention
The invention is directed to a device for the ablation of material by means of laser radiation, especially for treatment of biological substances in stomatologic, endoscopic, dermatologic, rhinoplastic and the like procedures, comprising a pump laser for generating a radiation-exciting laser radiation and comprising a laser converter for converting the pump laser radiation into a laser radiation that is provided for treatment in the wavelength range of 3 .mu.m.+-.0.2 .mu.m, wherein the laser converter is accommodated in a treatment head which is spatially separated from the pump laser, and wherein a fiber-optic device is provided for transmitting laser radiation from the pump laser to the laser converter.
b) Description of the Related Art
Devices of the type mentioned above belong, in principle, to the prior art and are known, for example, from DE OS 4341108 and EP 0530574. DE OS 4341108 describes a two-stage laser system for hard-tissue surgery in which the radiation of a first laser is transmitted, via an optical waveguide, to a second, miniaturized laser crystal and is used therein for coaxial pumping of this miniaturized laser crystal. The pump laser is a pulsed diode laser with an emission wavelength between 900 nm and 1000 nm. A fused-quartz fiber is used to transmit the pump laser radiation. The second laser which emits the actual working radiation is a rod-shaped erbium-yttrium-aluminum-garnet laser crystal. This YAG laser is arranged directly in a handpiece from which the working radiation can be directed to the hard dental tissue, namely at a wavelength in the range of 3 .mu.m.
Three different variants are indicated for the arrangement of the pump laser and the arrangement of the laser inserted in the handpiece. A first variant provides for the use of a Nd:YAG laser with a pulse energy of approximately 2 Joules and a pulse width of about 1 ms to 50 ms. This YAG radiation is transmitted via a fused-quartz fiber with a core diameter of approximately 80 .mu.m to an Er:YAG laser crystal arranged in the handpiece. The Er:YAG crystal is doped with ytterbium atoms. In a second embodiment form, a Nd:YAG laser emitting a wavelength of 1.44 .mu.m is used as pump radiation source. The 1.44-.mu.m radiation is again transmitted through a fused-quartz fiber to the dental handpiece and is used for pumping an Er:YGD laser crystal. In the third embodiment form, laser radiation is first generated with a wavelength of 2.65 .mu.m to 2.69 .mu.m and is transmitted via extremely anhydrous fused-quartz fibers in the handpiece and used for pumping an Er:YAG laser crystal.
A disadvantage in all of the suggested solutions consists in the very high thermal losses in the conversion of the pump radiation into the working radiation which must be compensated through relatively large geometric dimensions of the laser crystal in the handpiece. Therefore, the handpiece in which the laser crystal is integrated must necessarily also have large dimensions. Further, the output losses require that the pump laser radiation must be transmitted with very high intensity through the optical fiber to the handpiece, which leads to overloading of currently available optical fiber materials, so that their possible useful life is reduced to an undesirable extent.
The second reference cited above proposes a process and a device for ablation of biological hard substance, especially hard dental substance, which use a short pulsed laser in the wavelength range between 2.78 .mu.m and 2.94 .mu.m. Further, means are provided which even out the intensity time response of the laser radiation, so that transmittability via the optical waveguide is improved.
The short pulsed laser is accommodated in the handpiece as a solid-state laser. A Cr-Er-doped YSGG (yttrium-scandium-gadolinium-garnet) crystal is preferably used as laser crystal. Alternatively, it is suggested to use an Er:YAG laser which emits radiation in the wavelength range of 2.96 .mu.m. This makes use of the fact that the absorption maximum of the target substance shifts dynamically toward shorter wavelengths because of the absorption process during the active period of the pulsed laser radiation itself. When treating hard dental substance, for example, the absorption maximum of water-containing hard dental substance (hydroxyapatite) shifts from approximately 3 .mu.m to approximately 2.8 .mu.m. The transmittability of the pump laser radiation via optical fibers to the laser crystal in the handpiece is improved in that an additional crystal with light-linear optical characteristics is placed in the beam waist of the laser, either inside or outside of the resonator. According to the reference, lithium iodate or silver-gallium-sulfide is used for this crystal.
In addition, optical steps are provided for reducing the radiation loss and accordingly the output loss in comparison with the prior art, e.g., the formation of the resonator from 2 confocal mirrors, between which a laser-active medium (gas, liquid, solid or semiconductor) is arranged. The crystal for smoothing the intensity time response is arranged inside the resonator.
This suggestion still has the disadvantage of high output losses calling for excessively large geometric dimensions of the laser converter and accordingly also of the handpiece, so that additional areas of application in medicine, e.g., ablation not only in the field of stomatology but also in endoscopic laser therapy, rhinoplasty, etc., could not be fully developed.
However, it is desirable that these areas be developed because it is known that not only hard biological substances such as hard dental tissue, but also other materials containing H.sub.2 O and OH groups show an absorption maximum of radiation in the wavelength range of 2.8 .mu.m to 3.1 .mu.m and can be treated by means of this radiation effectively and without negative secondary effects, since laser treatment at a wavelength of 3 .mu.m has the decisive advantage over treatment at other wavelengths that the penetration depth is smaller and the negative effect on deeper layers is reduced. When the geometric dimensions of the handpiece are reduced, additional very useful possibilities for application beyond the aforementioned areas of human medicine extend to veterinary medicine and, finally, in general, to the treatment of other organic and inorganic materials under difficult conditions of access.
Thus, there is an extensive need to open up these areas of application with respect to technical instrumentation. While the exciting laser radiation can be transmitted along sufficiently long lengths to a treatment head in the aforementioned solutions through available fused-quartz fibers, it is necessary, in order to make the device usable for the above-mentioned areas of application, that the laser crystal in the laser converter and the treatment head in which the laser converter is accommodated are produced with sufficiently small dimensions while retaining a sufficiently high laser output, which in turn only succeeds when the device has a very high efficiency or when the thermal losses in the laser converter can be kept very low. Therefore, the energy efficiency of the laser crystal in the laser converter becomes a decisive characteristic.